1. Field of the Invention
The present invention relates to an optical coherence tomographic imaging apparatus and an optical coherence tomographic imaging method. In particular, the present invention relates to an optical coherence tomographic imaging apparatus and an optical coherence tomographic imaging method for acquiring a tomographic image of an eye fundus, a skin, and the like by optical coherence tomography, and to a program for executing the optical coherence tomographic imaging method and a storage medium having the program stored thereon.
2. Description of the Related Art
In recent years, an optical coherence tomographic imaging apparatus (optical coherence tomography apparatus: hereinafter, referred to as “OCT apparatus”) that employs an optical interference technology utilizing low-coherent light has been put into practical use. The OCT apparatus is an apparatus that is useful in a medical field, in particular, in an ophthalmologic field. The OCT apparatus is capable of acquiring a tomographic image of a portion of a retina in an eye fundus, and has therefore gradually been regarded as an apparatus indispensable for diagnosing illnesses in an eye fundus portion.
The principle of the OCT apparatus is briefly described. Low-coherent light is divided into reference light and measuring light. The measuring light is caused to enter an object to be inspected, and return light, which is reflected from an area to be imaged as a tomographic image, is caused to interfere with the reference light, with the result that a tomographic image of the object to be inspected can be acquired. The OCT apparatus is classified into two types, that is, a time domain (TD) type and a Fourier domain (FD) type. The FD-OCT apparatus determines a reflection intensity profile with respect to a distance (depth) by performing Fourier transform for an acquired interference signal with respect to a wavenumber. Through scanning of an irradiated portion of the object to be inspected, the tomographic image can be acquired. The FD-OCT apparatus can acquire the tomographic image at higher speed as compared to the TD-OCT apparatus, and hence the FD-OCT apparatus has become the current mainstream.
As a method of image quality evaluation for the OCT tomographic image, for example, there are provided graininess evaluation using a signal-to-noise ratio (SNR), a contrast-to-noise ratio (CNR), and a Wiener spectrum, and resolution evaluation using a modulation transfer function (MTF). Those indices are physical indices focusing on part of the image quality.
On the other hand, as an evaluation index reflecting the subjectivity of end users (for example, doctor and clinical technologist), a quality index (QI) is disclosed in D. M. Stein, J. G. Fujimoto, et al. Br. J. Ophthalmol. 2006 February; 90(2): 186-190. The QI is an image quality evaluation index determined based on a histogram for a luminance value of an image, and it is reported that the QI has a higher correlation with the evaluation of the end users as compared to the physical indices such as the SNR.
FIG. 8 is an illustration of a histogram showing the QI. The QI is expressed by the following expressions.
                    QI        =                  IR          ×          TSR                                    (                  Expression          ⁢                                          ⁢          1.1                )                                IR        =                                            (                              Saturation                -                low                            )                        Low                    ×          100                                    (                  Expression          ⁢                                          ⁢          1.2                )                                TSR        =                              Number            (                          Saturation              ⁢                              ∼                            ⁢              Middle                        )                                Number            ⁡                          (                              Middle                ⁢                                  ∼                                ⁢                Noise                            )                                                          (                  Expression          ⁢                                          ⁢          1.3                )            
Note that, in D. M. Stein, J. G. Fujimoto, et al. Br. J. Ophthalmol. 2006 February; 90(2): 186-190, “Saturation”, “Low”, “Noise”, and “Middle” are defined as follows.
Saturation: 99th percentile of the histogram
Low: 1st percentile of the histogram
Noise: 75th percentile of the histogram
Middle: average value between “Saturation” and “Noise”
“Number(Saturation˜Middle)” represents the total number of pixels having a luminance value in a range of from “Saturation” to “Middle” in the histogram. “IR” is a term corresponding to the signal-to-noise ratio (SNR), and “TSR” represents, as shown in FIG. 8, a ratio of the number of pixels in a bright layer to the number of pixels in a dark layer.
Further, as one of the OCT imaging methods in ophthalmology, imaging using an enhanced depth imaging (hereinafter, referred to as “EDI”) method is known. The EDI method is a method to be used mainly for observing details of a choroid with a focus on a relationship between the choroid and the illness. The EDI method is a method of acquiring a tomographic image as a reverse image under a state in which a position of a coherence gate is situated more deeply than the position of the choroid. The coherence gate represents a position at which the optical distances of the measuring light and the reference light in the OCT apparatus are equal to each other.
In recent years, as described in, for example, IMAMURA, YUTAKA et al. Retina, Vol. 29, pp. 1469-1473 (2010), studies on the choroid thickness of affected eyes have been proceeding, and hence the significance of diagnosing the choroid has been increasing.
The choroid is a layer which is low in luminance (low in regular reflectance), and hence, as a portion of the retinal image which has a low luminance value is rendered with higher accuracy, the user can diagnose the choroid with higher accuracy. However, the image quality evaluation index described in D. M. Stein, J. G. Fujimoto, et al. Br. J. Ophthalmol. 2006 February; 90(2): 186-190 is not appropriate as an index for evaluating the image quality of an image acquired by the EDI method because the rendering of a portion of the image which has a high luminance value is rated high.